Ultrasonic imaging systems are known to the prior art for providing real-time, cross-sectional images of human cardiac, abdominal and peripheral vascular structure that are of substantial diagnostic value. While various types of image formats have been provided, one of the more useful image formats from a diagnostic standpoint is the two-dimensional sector image which comprises an image of those body tissues located within a substantially planar sector. To develop the information required for a two-dimensional sector image, the ultrasonic imaging system includes a scanner which transmits a plurality of bursts of ultrasonic enery which propagate outwardly from a common point of origin in a plurality of angularly spaced-apart, radially-directed beams, and which detects, for each beam, return or echo pulses that occur when the associated burst of ultrasonic energy is scattered or reflected by tissue interfaces that lie in the beam. Since the time which elapses between the transmission of each ultrasonic energy burst and reception of any associated return pulse is related to the distance between the tissue interface causing the return pulse and the common point of origin, the return pulses can be processed to provide the two-dimensional sector image.
Such scanners may provide either a mechanical or an electronic scan of the sector. Mechanical scanners can be visualized as including a scan head which is maintained in contact with the skin of the body and which contains a continuously-rotating or oscillating ultrasonic transducer that is enabled for signal transmission and reception while the transducer is traversing a desired sectorial angle. When enabled; the transducer is caused to transmit a burst of ultrasonic energy and receive any associated return pulses at each of a plurality of incremental angular positions (or scan lines) thereof. The output signals provided by such mechanical scanners are in analog form and are referenced to a polar coordinate system wherein: the transmitted ultrasonic energy can be mathematically modeled as a point source that is located at the origin of the polar coordinate system; the value of the angular or azimuthal coordinate, .theta. expresses the direction of any transmitted ultrasonic energy burst and its associated return pulses (or, the relative angular position of any scan line); and, the value of the radial or axial coordinate, r, expresses the radial distance or range between the point of origin and the tissue interfaces causing the return pulses along any scan line.
In order that the output signals from the scanner may be visually displayed, in real-time, by a conventional video display apparatus whose scan is based on a Cartesian coordinate system and whose scanning rate may differ from that of the scanner, the output signals from the scanner are buffered and converted to Cartesian coordinates by a scan converter such as that described and claimed in U.S. Pat. No. 4,214,269, Parker et al., REAL TIME DIGITAL SCAN CONVERTER, issued July 22, 1980 to the assignee of the present invention.
While mechanical scanners provide acceptable operation and in fact are used in the majority of medical ultrasonic imaging systems providing two-dimensional sector images, they are subject to certain disadvantages. For example, the mechanical components of the scanner experience wear during operation with resultant degradation in image quality and eventual failure. As another example, the ultrasonic energy transmitted by the single transducer can only be focused within a limited range. As yet another example, the point of origin of the scan lines in a mechanical scanner lies at the transducer pivot and thus behind the interface between the scan head and the skin, so that the scanner must be carefully designed to minimize the distance between the point of origin and the skin.
In order to overcome the noted disadvantages of mechanical scanners, electronic or "phased array" scanners have been proposed which also operate to provide output signals in polar coordinate format and in analog or digital form for scan conversion and resultant display. Such electronic scanners include no moving mechanical components and theoretically can provide excellent axial and azimuthal resolution, dynamic focusing along each scan line, a common point of origin of the scan lines which is at the skin, and flexible beamforming which can be used to develop a wide variety of sector formats and other image formats. As will be apparent from the ensuing discussion, however, the electronic scanners heretofore known have been difficult and costly to implement in any practical form which provides acceptable image quality, and therefore have not found widespread acceptance.
Referring now to FIG. 1, the functional representation therein of a prior art electronic scanner includes a plurality of identical ultrasonic transducers X1-X12 arranged in a linear array, with each transducer being equally spaced from its adjacent transducers in the array. Each transducer is of the type which is capable of substantially omnidirectional ultrasonic energy transmission and reception at a frequency f.sub.0, with ultrasonic energy from and to the back side of each transducer being absorbed (by means not illustrated). In order to scan an object point in front of the array which is located at infinity and which lies along a line perpendicular to the array (.theta.=0), electrical pulses are simultaneously applied to the transducers, resulting in the simultaneous transmission of acoustic pulses therefrom which combine to form a transmitted beam of ultrasonic energy having substantially the same characteristics as a beam transmitted by a single ultrasonic transducer whose dimensions, or aperture, are the same as those of the array. In order to steer the scan to an object point which is located at infinity and which lies on a scan line angularly disposed by an azimuthal angle .theta., the electrical pulses applied to the transducers must have a predetermined timing relationship to each other that results in the propagation of a substantially planar acoustic wave front along the desired scan line.
Return pulses from any object point at infinity will combine in a substantially planar acoustic wave front propagating toward the array along the scan line so as to result in electrical signals being produced by the transducers that have substantially the same timing relationship to each other as the electrical pulses used to excite the transducers. The electrical signals representing return pulses accordingly must be processed in an appropriate manner so that simultaneous detection of all return pulses from a given object point can be made.
It can be shown that, for an object point located at infinity and lying along a scan line at any angle .theta., the predetermined timing relationship between the "transmitted" electrical pulses and between the "received" electrical signals is linear. For an object point closer to the array, this linear relationship must be modified so as to result in the transmission and reception of beams having substantially circular acoustic wave fronts that converge about the object point.
Implementation of the aforementioned timing relationships is illustrated in FIG. 1 by a common node 10 that is coupled to each of the transducers X1-X12 through a corresponding one of a plurality of bidirectional parallel delay elements DL1-DL12, with the delay afforded by each delay element between a signal on common node 10 and its associated transducer, and vice-versa, being selectively adjustable. For an object point OP located at the right of the array and lying at a distance less than infinity from the array, it will be seen that, as schematically represented in FIG. 1, the application of a transmit signal T (comprising a burst of pulses at frequency f.sub.0) to common node 10 results in transducer X12, transducer X11, and the remaining transducers in the array being excited in succession, due to the increasing amounts of delay afforded by successive delay lines DL12, DL11, etc. The resultant acoustic pulses from the transducers combine in a beam having a substantially circular acoustic wave front WF which converges about object point OP. Likewise, returns of the transmitted ultrasonic energy from object point OP combine to form a substantially circular acoustic wave front identical in form to wave front WF which propagates back toward the array and which reaches, in succession, transducer X1, transducer X2, and so forth down the array to transducer X12. It will be noted that the same delays given to the transmit signal T by delay elements DL1-DL12 are also given to the received electrical signals from transducers X1-X12, so that the received electrical signals all arrive at common node 10 at substantially the same time. Accordingly, there appears on common node 10 a receive signal R representing all returns of the transmitted ultrasonic energy from object point OP. It can be appreciated that, through selective adjustment of the delays afforded by delay elements DL1-DL12, the transmitted and received beams of the scanner can be steered to any desired azimuthal angle and can be focused to any desired range at each azimuthal angle.
For beam transmission, digital timing circuitry is conventionally used, rather than the delay elements illustrated in FIG. 1, to generate the electrical signals applied to the transducers of the array. In many cases, precise focusing of the transmitted beam is not necessary, resulting in simplification of the digital timing circuitry. The received electrical signals from the transducers, being analog in nature and of wide dynamic range, are not so easily treated and it heretofore has been thought necessary to utilize some sort of delay element or elements to effect processing and detection of the received electrical signals. In a straightforward implementation of the parallel approach illustrated in FIG. 1, each delay element includes an input (which is coupled to its associated transducer) and a plurality of outputs or taps, with each delay element functioning to provide signals on its taps which are delayed from the signal on its input by predetermined and different amounts. In order to effect steering and focusing of the received beam, a switching apparatus is selectively controlled to connect a tap on each delay element to a common summing junction. For a typical array including thirty-two transducer elements operating at a nominal frequency of 2.5 MHz, it can be shown that in order to obtain phase coherence of the received electrical signals that is within .lambda./8 (at frequency f.sub.0), approximately 160 taps and a maximum delay of approximately eight microseconds are required for each delay element. It is difficult, and accordingly expensive, to fabricate such electrical delay elements which can provide acceptable operation, given the large number of taps, the high frequency of operation, and the maximum delay that is required. The switching apparatus used to connect the taps of the delay elements to the common summing junction is also necessarily complex and expensive. For these reasons, electronic scanners following a straightforward implementation of the parallel approach in FIG. 1 have not been commercially implemented.
A number of approaches have been taken in the prior art to reduce the number of taps required for each delay element and to reduce the maximum delay that must be afforded thereby.
One of these approaches can be seen in U.S. Pat. No. 4,005,382, Beaver, in which it is recognized that although there is a maximum delay that must be provided in detection of the received signals (e.g., the delay that is required between the received electrical signals at the rightmost transducer and the received electrical signal at the leftmost transducer when the ultrasonic beam is steered to either the far-right or the far-left), the delay between the received electrical signals from adjacent transducers is much less. Accordingly, the scanner in the Beaver patent includes a plurality of short adjustable delay elements, each associated with a transducer of the array, and a plurality of selectively-actuable switches for interconnecting adjacent ones of the delay elements. The switches function to connect the input of a given delay element to the output of the adjacent delay element, to connect the output of a given delay element to the input of the adjacent delay element, or to connect the output of a given delay element to the output of the adjacent delay element. To give an example of the operation of this electronic scanner, let it be assumed that the received beam is to be steered and focused to the right of the array. In such a case, the switches are actuated so that the output of the rightmost delay element is connected to the input of the adjacent delay element to the left in the array, the output of the adjacent delay element is connected to the input of the next-adjacent delay element to the left in the array, and so forth, resulting in a transverse "pipeline" structure. The received electrical signal from the rightmost transducer is therefore delayed by its associated delay element, and summed with the received electrical signal from the adjacent transducer to the left in the array. The thus-summed pulses are then delayed in the delay element associated with the next adjacent transducer, and then summed with the received electrical signal from the next-adjacent transducer, and so forth. An electrical signal corresponding to the received acoustic wave front therefore passes transversely across the array until the contribution to that signal from the leftmost transducer has been made. For beam steering and focusing to object points lying directly ahead of the array and closer than infinity, the switches are actuated so that the received electrical signal from the right centermost element in the array passes to the right in the array through succeeding delay elements and the received electrical signal from the left centermost element in the array passes to the left in the array through successive delay elements.
While the approach taken in the Beaver patent significantly reduces the length or maximum delay of each delay element and the number of discrete delay values that must be provided in each delay element, it is subject to serious disadvantages. For example, if the beam is steered and focused to either side of the array, the received electrical signals from the transducers on that side of the array must pass through a greater number of switches and delay elements than the received electrical signals from the transducers on the other side of the array. Since the received electrical signal from each transducer accordingly passes through a different number of delay elements, the received electrical signals are distorted with respect to each other. To minimize this distortion, it has been found necessary to limit the number of transducers used in the array, with a resultant loss of lateral resolution. As another example, the fact that two signals must be developed for object points directly ahead of the array and closer than infinity (i.e., signals passing to the right and to the left in the array) result in a loss of image quality for such object points.
Another approach is found in the electronic scanner taught in U.S. Pat. No. 4,019,169, Takamizawa. In this patent, each delay element in the parallel approach illustrated in FIG. 1 comprises a plurality of capacitors, a plurality of write switches for coupling the received electrical signal from the associated transducer to the capacitors, and a plurality of read switches for coupling the capacitors to a common summing junction for all of the delay elements. The write switches are successively actuated at a frequency (e.g., 8 MHz) so as to sample successive amplitude levels of the received electrical signal from the associated transducer and to accordingly store the sampled signal levels on the capacitors. The capacitors are then read by successive actuation of the read switches, at a time corresponding to the desired delay. While the approach taken in the Takamizawa patent is meritorious if the sampling rate can be made high enough, the fact remains that the sampling rate is limited by the operational speeds of currently-available switching transistors. Accordingly, the received electrical signal stored in the capacitors is distorted and loss of image quality results.
A digital version of the electronic scanner in the Takamizawa patent has also been proposed, wherein each delay element includes an analog-to-digital converter, a random access memory (RAM), and a digital-to-analog converter. The received electrical signal from each transducer is digitized by the analog-to-digital converter, stored in appropriate locations in the RAM, and read from the RAM at the appropriate delay time and reconverted to analog form by the digital-to-analog converter. In order to reduce the amount of digital data that is stored in the RAM, a limited number (e.g., sixteen) of amplitude levels are used to digitize the received electrical signal from the transducer, thereby resulting in coarse quantization and consequent loss of image quality.
In U.S. Pat. No. 4,155,260, Engeler et al., another approach is taught which avoids the problems associated with the high signal processing rate taught in the Takamizawa patent. In this patent, each received electrical signal is synchronously demodulated by mixing the output signal from the transducer with a reference signal that preferably has a frequency equal to that of the transmitted ultrasonic energy and that has a predetermined phase relationship to the output signal from the transducer. The mixer output is filtered and the resultant filtered signal is applied to a delay element, with the outputs of the delay elements for all of the transducers being summed to provide the desired output signal. Since the filtered signal is at a lower frequency than the received electrical signal from the transducer, relatively inexpensive and available charge-coupled devices can be used in each delay element, with the charge-coupled devices being arranged in a manner similar to an analog shift register in which the filtered signal passes through the charge-coupled devices from the input of the delay element to the output thereof at a rate determined by a clock signal applied to the charge-coupled devices. Although the approach taken in the Engeler et al. patent is advantageous in that the scanner may be adapted to different frequencies of ultrasonic energy transmission and reception by simply changing the frequency of the reference signal, it is subject to certain disadvantages primarily resulting from the use of the delay elements therein. Each delay element includes a large number of components, and the filtered signal passing therethrough is distorted due to attenuation and switching transients which result in degradation of image quality.
The approach taught in U.S. Pat. No. 4,140,022, Maslak, is notable for its recognition that the delay that must be afforded to each received electrical signal can be subdivided into a "fine" delay which is achieved by phase-shifting of the received electrical signal and a "coarse" delay which is achieved through the use of a delay element or elements. Phase-shifting is accomplished by passing the output signal from each transducer through a phase shifter which is set to provide the required amount of "fine" delay, or by mixing the output signal from each transducer with a reference signal whose frequency is chosen to yield a desired intermediate frequency in the mixer output and whose phase is chosen to yield a desired phase shift in the mixer output. The output signals from the phase shifters or mixers are then either applied to individual delay elements for each transducer, with the taps on the individual delay elements being connected by appropriate switches to a common summing junction (in a manner similar to the "parallel" approach illustrated in FIG. 1), or, connected through appropriate switches to respective taps of a master delay element having a single output (a "transverse" approach similar to that previously discussed for the Beaver patent). Due to the "fine" delay that has been achieved by phase-shifting, the number of taps on either the individual delay elements or the master delay element are significantly less than those required for the approach illustrated in FIG. 1, and the incremental delay values between adjacent taps are much greater. Accordingly, inexpensive electrical or acoustic delay lines may be used for the individual delay elements or the master delay element. Further, focusing can be readily achieved by adjusting the phase shift provided by either the phase shifters or the mixers.
The primary disadvantage of the approach taken in the Maslak patent again results from distortion that is occasioned by the use of delay elements. That is, signals passing through the delay elements are attenuated, and signals coupled into and out of the delay elements by switches are distorted by switching transients.
It is therefore an object of this invention to provide an improved electronic or phased array scanner for ultrasonic imaging apparatus.
It is another object of this invention to provide an improved beamforming apparatus and method for use in such a scanner.
It is yet another object of this invention to provide such a beamforming apparatus and method which does not require the use of any delay elements for the signals received by the transducers of the array.
It is still another object of this invention to provide an improved beamforming apparatus and method for electronic scanners that is particularly adapted to provide an output signal useful in developing a two-dimensional sector image.
It is a further object of this invention to provide a beamforming apparatus and method that can be implemented by the use of a minimum number of readily-available, inexpensive components.
It is still a further object of this invention to provide a beamforming apparatus and method that functions to provide a two-dimensional sector image of excellent image quality and resolution.